Catalyst electrode layer, membrane-electrode assembly, and fuel cell

ABSTRACT

A catalyst electrode layer is configured to be disposed in contact with an electrolyte membrane of a fuel cell. A content of Fe per unit area of the catalyst electrode layer is equal to or larger than 0 μg/cm 2  and equal to or smaller than 0.14 μg/cm 2 , and a water absorption rate of the catalyst electrode layer is equal to or higher than 11% and equal to or lower than 30%.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2014-217152 filed onOct. 24, 2014 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a catalyst electrode layer, amembrane-electrode assembly, and a fuel cell.

2. Description of Related Art

A membrane-electrode assembly used in a polymer electrolyte fuel cellincludes a proton-conducting polymer electrolyte membrane, and an anodeand a cathode that are catalyst electrode layers provided on theelectrolyte membrane. In the fuel cell, hydrogen or oxygen supplied tothe membrane-electrode assembly may pass through the electrolytemembrane without being used for a power generation reaction, and maymove to the electrode opposite to the electrode to which the hydrogen oroxygen is supplied. In this case, hydrogen peroxide (H₂O₂) may begenerated at the electrode side where there are hydrogen and oxygen. Itis known that the catalyst electrode layer is deteriorated by hydrogenperoxide radicals generated from hydrogen peroxide. Thus, JapanesePatent Application Publication No. 2013-069534 (JP 2013-069534 A)describes a fuel cell including a separator in which a humidifyingpassage for supplying water to a catalyst electrode layer is formed inorder to discharge generated hydrogen peroxide radicals using the water.

However, there is room for improvement in the technology for suppressingthe deterioration of the catalyst electrode layer. The inventors of theinvention have found that, in order to improve the durability of themembrane-electrode assembly, it is more preferable to make the waterabsorption ability of the catalyst electrode layer fall within aprescribed range, in addition to forming the fuel cell such that thecatalyst electrode layer is maintained in a moist state during powergeneration of the fuel cell as in the above-mentioned related art.

SUMMARY OF THE INVENTION

An aspect of the invention relates to a catalyst electrode layerconfigured to be disposed in contact with an electrolyte membrane of afuel cell. A content of Fe per unit area of the catalyst electrode layeris equal to or larger than 0 μg/cm² and equal to or smaller than 0.14μg/cm², and a water absorption rate of the catalyst electrode layer isequal to or higher than 11% and equal to or lower than 30%. The waterabsorption rate satisfies a relationship of the water absorptionrate=(Q3−Q1)/Q1×100−(Q2−Q1)/Q1×100, where Q1 is a weight of the catalystelectrode layer after the catalyst electrode layer is dried for 1 hourunder an environment in which a temperature is 100° C. and a relativehumidity is 0%, after a fuel cell including the catalyst electrode layeris maintained for 100 hours under a condition that a cell temperature is60° C., the relative humidity is 40%, and a generated voltage is 0.5 V,Q2 is a weight of the catalyst electrode layer after the catalystelectrode layer is further maintained for 1 hour under an environment inwhich the temperature is 70° C. and the relative humidity is 15%, and Q3is a weight of the catalyst electrode layer after the catalyst electrodelayer is further maintained for 1 hour under an environment in which thetemperature is 70° C. and the relative humidity is 90%. With thisconfiguration, it is possible to improve the durability of the catalystelectrode layer.

The invention may be implemented in various aspects. For example, theinvention may be implemented as a membrane-electrode assembly includingthe catalyst electrode layer, a fuel cell including themembrane-electrode assembly, and production methods thereof, andfurther, a production method for the membrane-electrode assemblyincluding the above-mentioned testing method.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is an explanatory diagram showing a schematic configuration of afuel cell according to an embodiment of the invention;

FIG. 2 is a flowchart showing a production process for amembrane-electrode assembly according to the embodiment;

FIG. 3 is a relationship diagram showing a relationship between a waterabsorption rate and a performance decrease rate with regard to samples#1 to #11;

FIG. 4 is a relationship diagram showing a relationship between thewater absorption rate and a cell resistance with regard to the samples#1 to #6, and #9 to #11;

FIG. 5 is a relationship diagram showing a relationship between thewater absorption rate and an ionomer decomposition rate with regard tothe samples #1 to #6, and #9 to #11;

FIG. 6 is a relationship diagram showing a relationship between thewater absorption rate and a content of Fe with regard to the samples #3to #6, and #9 to #11;

FIG. 7 is a relationship diagram showing a relationship between thewater absorption rate and a relative humidity with regard to the samples#1 to #6, and #9;

FIG. 8 is a relationship diagram showing a relationship between thewater absorption rate and a gas diffusion resistance with regard to thesamples #3 to #9; and

FIG. 9 is a relationship diagram showing a relationship between thewater absorption rate and the performance decrease rate with regard tothe samples #12 to #16.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 is an explanatory diagram showing a schematic configuration of afuel cell 10 according to an embodiment of the invention. The fuel cell10 is a polymer electrolyte fuel cell. The fuel cell 10 has a stackstructure in which a plurality of unit cells 14 are stacked. In the fuelcell 10, each unit cell 14 is a unit module that generates electricpower. The unit cell 14 generates electric power by using anelectrochemical reaction between hydrogen gas and oxygen contained inair. Each unit cell 14 includes a power generation body 20, and pairedseparators 100 (an anode-side separator 100 an, and a cathode-sideseparator 100 ca). The power generation body 20 is sandwiched betweenthe separators 100. The power generation body 20 includes amembrane-electrode assembly (MEA) 23, and paired gas diffusion layers 24(an anode-side diffusion layer 24 an and a cathode-side diffusion layer24 ca) that are disposed on respective sides of the membrane-electrodeassembly 23. The membrane-electrode assembly 23 includes an electrolytemembrane 21, and catalyst electrode layers 22 (an anode 22 an and acathode 22 ca) provided on respective surfaces of the electrolytemembrane 21.

The electrolyte membrane 21 is a proton-conducting ion-exchange membraneformed of, for example, a fluorine resin. The electrolyte membrane 21exhibits good electric conductivity in a moist condition. As theelectrolyte membrane 21, for example, a solid polymer electrolytemembrane formed of a perfluorosulfonic acid polymer that has at leastone sulfo group (—SO₃H group) at a side chain end may be used. Morespecifically, as the electrolyte membrane 21, for example, afluorine-based sulfonic acid membrane, such as a Nafion membrane (117,Nafion is a registered trademark), Aciplex (a registered trademark), orFlemion (a registered trademark), may be used.

The anode 22 an of the catalyst electrode layers 22 functions as ananode electrode when the fuel cell 10 generates electric power. Thecathode 22 ca of the catalyst electrode layers 22 functions as a cathodeelectrode when the fuel cell 10 generates electric power. For example,each of the catalyst electrode layers 22 may include carbon particles (acatalyst support) supporting catalyst metal (for example, platinum) thatpromotes the electrochemical reaction, and a proton-conducting polymerelectrolyte (ionomer). As the ionomer, for example, a perfluorosulfonicacid polymer that has at least one sulfo group (—SO₃H group) at the sidechain end may be used. The ionomer included in the catalyst electrodelayer 22 may the same polymer as the ionomer included in the electrolytemembrane 21, or a polymer different from the ionomer included in theelectrolyte membrane 21. As the conductive support (as the catalystsupport), for example, carbon materials such as carbon black, carbonnanotube, and carbon nanofiber, and carbon compounds such as siliconcarbide may be used, in addition to the carbon particles. As thecatalyst metal, for example, platinum-alloy, palladium, rhodium, gold,silver, osmium, and iridium may be used, in addition to platinum.

Each of the catalyst electrode layers 22 is preferably configured suchthat the water absorption rate of the catalyst electrode layer 22 isequal to or higher than 11% and equal to or lower than 30% (i.e., thewater absorption rate is 11% to 30%). The water absorption rate is avalue indicating the water-absorbing performance of the catalystelectrode layer. In the case where the water absorption rate of thecatalyst electrode layer 22 is equal to or higher than 11%, even ifhydrogen peroxide is generated due to cross leakage or the like, thegenerated hydrogen peroxide can be discharged by water contained in thecatalyst electrode layer 22. Therefore, decomposition of the ionomer dueto the hydrogen peroxide is suppressed. Further, in the case where thewater absorption rate is equal to or lower than 30%, it is possible tosuppress a decrease in the efficiency in supplying the gas to thecatalyst due to closure of pores of the catalyst electrode layer 22caused by swelling of the ionomer.

The amount of water absorbed into the catalyst electrode layer 22 varieseven under the same humidity environment (moisture environment),depending on, for example, the properties of molecules constituting theionomer. More specifically, the molecules constituting the ionomerinclude a perfluorocarbon-based main chain, and at least one side chainhaving a sulfo group (—SO₃H group) at the end thereof. In this case, theamount of water absorbed into the catalyst electrode layer 22 varieseven under the same humidity environment, depending on, for example, thenumber of the sulfo groups (—SO₃H groups) in the side chains, and therigidity of the main chain. The water absorption rate of the catalystelectrode layer 22 is influenced by (varies depending on) the kind andweight percent of the ionomer in the catalyst electrode layer 22, thekind and weight percent of the carbon (the catalyst support) in thecatalyst electrode layer, and the structure of the catalyst electrodelayer. For example, with regard to the ionomer, as the amount ofsulfonic acid increases, the water absorption rate increases, and as thecrystallinity increases, the water absorption rate decreases. Forexample, with regard to the carbon, as the surface area or the porevolume increases, the water absorption rate increases. Further, forexample, with regard to the catalyst layer structure, as the thicknessincreases, the water absorption rate increases. By adjusting thesefactors, the water absorption rate of the catalyst electrode layer 22can be set to a value in the above-mentioned range.

The water absorption rate of the catalyst electrode layer 22 iscalculated by the method described below. First, the fuel cell includingthe membrane-electrode assembly in which the catalyst electrode layers22 are formed is maintained for 100 hours under the condition that thecell temperature is 60° C., the relative humidity is 40%, and thegenerated voltage is 0.5 V. Then, the catalyst electrode layer 22 isscraped out from (taken out from) the membrane-electrode assembly, andis dried for 1 hour under the environment in which the temperature is100° C. and the relative humidity (RH) is 0%, and then, a weight Q1 ofthe catalyst electrode layer 22 is measured. Next, the catalystelectrode layer 22 is maintained for 1 hour under the environment inwhich the temperature is 70° C. and the relative humidity (RH) is 15%,and then, a weight Q2 of the catalyst electrode layer 22 is measured.Further, the catalyst electrode layer 22 is maintained for 1 hour underthe environment in which the temperature is 70° C. and the relativehumidity (RH) is 90%, and then, a weight Q3 of the catalyst electrodelayer 22 is measured. By using the expression (1) described below, thewater absorption rate of the catalyst electrode layer 22 is calculatedfrom the measured weights Q1, Q2, and Q3.

Water absorption rate=(Q3−Q1)/Q1×100−(Q2−Q1)/Q1×100  (1)

Iron (Fe) may be mixed into the catalyst electrode layer 22 depending ona production process or the like. In this case, it is preferable thatthe content of Fe should be equal to or larger than 0 μg/cm² and equalto or smaller than 0.14 μg/cm² (i.e., the content of Fe should be 0μg/cm² to 0.14 μg/cm²). In the case where the content of Fe is in theabove-mentioned range, even if hydrogen peroxide is generated, theionomer is unlikely to be decomposed by the generated hydrogen peroxide,as compared to the case where the content of Fe in the catalystelectrode layer is larger than 0.14 μg/cm².

The gas diffusion layers 24 are layers in which the reaction gas used inelectrode reactions (i.e., anode gas and cathode gas) is diffused alonga planar direction of the electrolyte membrane 21. Each of the gasdiffusion layers 24 is formed of a porous gas diffusion layer basematerial. As the gas diffusion layer 24, for example, a carbon porousbody formed of carbon paper or carbon cloth may be used. Awater-repellent layer may be formed in the gas diffusion layer 24 suchthat the gas diffusion layer 24 has a water-repellent property, bycoating the gas diffusion layer base material with water-repellent paste(i.e., by performing water-repellent treatment on the gas diffusionlayer base material). As the water-repellent paste, for example, a mixedsolution of carbon powder and a water-repellent resin (for example,polytetrafluoroethylene (PTFE), polyethylene, or polypropylene) may beused.

Each of the separators 100 is formed of a member that has a gas barrierproperty and electron conductivity. For example, the separator 100 isformed using a carbon member made of, for example, dense carbon that ismade gas-impermeable by compressing carbon, or a metal member that ismade of stainless steel or the like, and that is formed by a pressingprocess. Protrusions and recesses are provided on the surface of theseparator 100 so as to form flow passages through which the gas andliquid flow. Anode gas passages AGC are formed between the anode-sideseparator 100 an and the anode-side diffusion layer 24 an. Cathode gaspassages CGC are formed between the cathode-side separator 100 ca andthe cathode-side diffusion layer 24 ca.

FIG. 2 is a flowchart showing a production process for themembrane-electrode assembly 23 according to the embodiment. In order toproduce the membrane-electrode assembly 23, first, catalyst ink isproduced (step S100). More specifically, an ionomer that is aperfluorosulfonic acid polymer that has at least one sulfo group (—SO₃Hgroups) at the side chain end, and catalyst-supporting carbon areprepared. Then, the ionomer and the catalyst-supporting carbon aredispersed in an aqueous solution of a solvent (for example, alcohol) toproduce the catalyst ink. The operation of dispersing the ionomer andthe catalyst-supporting carbon in step S100 is not particularly limited,as long as the ionomer and the catalyst-supporting carbon can besufficiently dispersed in the solvent. For example, processes such as astirring process and an ultrasonic process may be appropriatelycombined.

The catalyst-supporting carbon can be produced, for example, bydispersing carbon particles made of carbon black in a solution of aplatinum compound, and performing an impregnating process, acoprecipitation process, or an ion exchange process. As the solution ofthe platinum compound, for example, a solution of a tetraammine platinumsalt, a solution of dinitrodiammine platinum, a solution of a platinumnitrate, or a solution of a chloroplatinic acid may be used. Forexample, the amount of the catalyst-supporting carbon mixed with theionomer is in a range such that the weight ratio of the ionomer withrespect to the catalyst-supporting carbon is 0.5 to 1.2.

After step S100, the produced catalyst ink is applied onto a base plate,and is dried (step S110). The base plate is not particularly limited, aslong as a membrane can be formed by applying the catalyst ink onto thebase plate. The base plate may be a thin membrane formed of, forexample, polyethylene terephthalate (PET) or polytetrafluoroethylene(PTFE). The method of applying the catalyst ink in step S110 is notparticularly limited. For example, a spray method, a screen printingmethod, a doctor blade method, or a die coating method may be employed.By applying the catalyst ink onto the base plate, and drying thecatalyst ink, the solvent in the catalyst ink vaporizes, and thus, thelayer of the catalyst ink (the catalyst ink layer) becomes a porouslayer.

Then, the catalyst ink layer on the base plate is heated such that abase plate-side of the catalyst ink layer (i.e., a side of the catalystink layer, which is in contact with the base plate) is a hightemperature side (step S120). More specifically, heating is performedsuch that the temperature of the base plate-side of the catalyst inklayer is a first temperature that is set in advance, and the temperatureof a side of the catalyst ink layer, which is not in contact with thebase plate, is a second temperature lower than the first temperature,that is, a temperature gradient is provided within the catalyst inklayer.

The catalyst ink layer heated in step S120 is transferred onto theelectrolyte membrane (step S130), and thus, the membrane-electrodeassembly is completed. The catalyst ink layer may be transferred ontothe electrolyte membrane, for example, by hot pressing while a surfaceof the catalyst ink layer, on which the base plate is not provided, isin contact with the electrolyte membrane. After the catalyst ink layeris transferred onto the electrolyte membrane, the base plate isseparated (removed) from the catalyst ink layer. Thus, the cathode isformed on the electrolyte membrane.

In the embodiment, the anode is formed on the electrolyte membrane byapplying the catalyst ink that is the same as or similar to the catalystink used to form the cathode, onto a base plate, and transferring thecatalyst ink layer onto the electrolyte membrane without performingheating in step S120. The anode may be formed by transferring thecatalyst ink layer onto the electrolyte membrane before step S130.Alternatively, the anode may be formed by transferring the catalyst inklayer onto the electrolyte membrane in step S130 after the cathode isformed by transferring the catalyst ink layer onto the electrolytemembrane.

In order to confirm the effect in the embodiment, sixteen samples #1 to#16 of the membrane-electrode assembly were prepared, and durabilityevaluation was performed on the catalyst electrode layer included ineach sample.

(1) Each of the samples #1 to #8 was produced as described below. Acatalyst was produced, catalyst ink was produced using the catalyst, acatalyst electrode layer was produced using the catalyst ink, and amembrane-electrode assembly (MEA) was produced using the catalystelectrode layer.

(Production of catalyst powder) As carbon for supporting the catalyst,acetylene black-based carbon was used. The acetylene black-based carbonhad the surface area of 850 m²/g, the primary particle size of 12 nm,the bulk density of 0.02/ml, the crystal size (La) of 20 nm, the iodineadsorption amount of 870 mg/g, and the dibutyl phthalate oil absorption(DBP oil absorption) of 280 ml/g. Then, 5.0 g of the acetyleneblack-based carbon was added to 1.2 L of pure water, and was dispersedin the pure water to produce a dispersion solution. A hexahydroxoplatinum nitrate solution containing 5.0 g of platinum, and a cobaltnitrate aqueous solution containing 0.21 g of cobalt were dropped intothe dispersion solution, and were sufficiently stirred with the carbon.Then, after the dispersion solution was stirred, approximately 100 ml of0.1N ammonia was added to the dispersion solution to achieve a pH ofapproximately 10. Thus, hydroxide was formed and was deposited on thecarbon. The dispersion solution was filtered to obtain powder, and theobtained powder was dried under vacuum at 100° C. for 10 hours. Next, areduction process was performed while the powder was maintained at 400°C. for 2 hours in hydrogen gas. Then, the powder was maintained at 1000°C. for 10 hours in nitrogen gas to produce alloy powder. Thus, catalystpowder was obtained. The catalyst powder was stirred in 1.0N nitric acidfor 2 hours. In the composition of the obtained catalyst, Pt was 49 wt%, Co was 2 wt %, and C was 49 wt %. The average particle size of PtCowas 4 nm.

(Production of catalyst ink) Then, 10 ml of ultrapure water was added to1 g of the produced catalyst powder, and stirring was performed. Then, 5ml of ethanol was added, and stirring was performed with the use of astirring rod to obtain a suspension in which particles were in a fullysuspended state. Then, an ionomer solution with an equivalent weight(EW) of 910 as an ion conductor was slowly dropped into the suspensionuntil the weight ratio of the solid content of the ionomer solution tothe carbon in the catalyst (hereinafter, referred to as “I/C”) became1.0, and was dispersed for 30 minutes with the use of an ultrasonicdispersion device to obtain uniform slurry. Thus, the catalyst ink asthe catalyst electrode material was produced.

(Production of catalyst electrode layer) The produced catalyst ink wasuniformly applied onto a sheet of Teflon (a registered trademark) withthe use of a squeegee such that the weight of platinum per unit area ofthe catalyst was 0.3 mg/cm². After the catalyst ink was applied onto theTeflon sheet, the Teflon sheet was dried at 80° C. for 3 hours, andthus, the catalyst electrode layer was produced.

(Production of membrane-electrode assembly) Nafion (the registeredtrademark) 117 was used as the solid polymer electrolyte membrane, andthe produced catalyst electrode layer was used as each of the anode andcathode. While the solid polymer electrolyte membrane was sandwichedbetween the anode and the cathode, hot pressing was performed at 170° C.for 300 seconds. Thus, the membrane-electrode assembly was produced.

(Durability test) The produced membrane-electrode assembly wassandwiched between gas diffusion layer base materials each of which wasformed of carbon fiber and a water-repellent layer. H₂ was supplied tothe anode, and air was supplied to the cathode, and current-voltagecharacteristics (i.e., I-V characteristics) were measured at the celltemperature of 60° C. The current value (A) at the cell voltage of 0.5 Vwas regarded as initial performance (performance before a durabilitytest). Then, the durability test was conducted. More specifically,electric power was generated by the samples #1 to #8 under the samecondition for 100 hours. The relative humidities (%) of the samples #1to #8 during the durability test were described below. The relativehumidity was measured at a gas supply port on the cathode side. Sincethe relative humidities of the samples #1 to #8 during the durabilitytest were made different from each other, the water absorption rates ofthe samples #1 to #8 were made different from each other. The relativehumidities of the samples #1 to #8 during the durability test were madedifferent from each other, in order to easily simulate the difference inthe water absorption rate (the water absorption performance) due to thedifference in the composition of the catalyst electrode layer. Actually,the water absorption rate can be adjusted by adjusting the compositionof the catalyst electrode layer. The relative humidity of the sample #1was 20%, the relative humidity of the sample #2 was 30%, the relativehumidity of the sample #3 was 40%, the relative humidity of the sample#4 was 60%, the relative humidity of the sample #5 was 80%, the relativehumidity of the sample #6 was 100%, the relative humidity of the sample#7 was 130%, and the relative humidity of the sample #8 was 200%.

(2) Each of the samples #9 to #11 was produced as described below. Whenthe catalyst ink was produced, iron (III) nitrate was added, and thus,the catalyst electrode layer was produced such that the catalystelectrode layer contained Fe. Thus, the catalyst electrode layer wasproduced. The content of Fe per unit area of the catalyst electrodelayer in each of the samples #9 to #11 was as described below. The otherproduction conditions were the same as those used when the samples #1 to#8 were produced. The relative humidity during the durability test was40% as in the case of the sample #3. The content of Fe in the sample #9was 0.14 μg/cm², the content of Fe in the sample #10 was 0.28 μg/cm²,and the content of Fe in the sample #11 was 0.56 μg/cm².

(3) Each of the samples #12 to #16 was produced as described below. Thesamples #12 to #16 are different from the samples #1 to #8 in thefollowing points. The relative humidity during the durability test was40% as in the case of the sample #3. With regard to the sample #12,tetrahydrofuran was added instead of ethanol in the process of producingthe catalyst ink. With regard to the sample #13, acetone was addedinstead of ethanol in the process of producing the catalyst ink. Withregard to the sample #14, the Teflon sheet, to which the catalyst inkwas applied, was dried under vacuum of −200 mmHg in the process ofproducing the catalyst electrode layer. With regard to the sample #15,ethanol was not added in the process of producing the catalyst ink. Withregard to the sample #16, a fluorine-based solvent with a high boilingpoint (ASAHIKLIN AC-6000 produced by Asahi Glass Co., Ltd.) was addedinstead of water and ethanol, in the process of producing the catalystink.

(Durability performance evaluation) After the above-mentioned durabilitytest, the I-V characteristics were measured, and the current value (A)at the cell voltage of 0.5 V was regarded as the performance after thedurability test. A performance decrease rate was calculated from themeasured initial performance and the measured performance after thedurability test, using the expression (2) described below. Further, theimpedance at the frequency of 1000 Hz was measured as a cell resistance(Ω·cm²), with the use of a Frequency Response Analyzer (FRA).

Performance decrease rate=(initial performance−performance afterdurability test)/initial performance×100  (2)

(Physical property (water absorption rate)) After the above-mentioneddurability test, the cathode catalyst electrode layer was scraped out(taken out) from each sample, and the weight Q1, the weight Q2, and theweight Q3 were measured. In addition, the water absorption rate (%) ofthe catalyst electrode layer of each sample was calculated using theabove-mentioned expression (1).

(Physical property (ionomer decomposition rate)) Heating was performedon the cathode catalyst electrode layer, which was taken out from eachsample, in the stream of N₂ such that the temperature increased to 500°C. at the increase rate of 1° C./min. The amount of desorbed sulfur (S)components of the ionomer was measured with the use of a mass analyzer.The ionomer decomposition rate (%) was measured based on the ratiobetween the amount of S components before the durability test and theamount of S components after the durability test, as indicated by theexpression (3) described below.

Ionomer decomposition rate=(the amount of S components before durabilitytest−the amount of S components after durability test)/the amount of Scomponents after durability test×100  (3)

(Physical property (gas diffusion resistance)) With regard to eachsample after the durability test, the I-V characteristics were measuredwhile the relative humidity was set to 30%, and the reaction gas wassupplied such that the oxygen concentration was low. The limitingcurrent (A) was measured based on the obtained I-V characteristics. Thelimiting current is a current in a portion in which the current does notincrease with a decrease in the voltage, in the I-V characteristics. Thegas diffusion resistance (sec/m) was calculated from the measuredlimiting current, using the expression (4) described below.

Gas diffusion resistance=O₂ partial pressure (Pa)×Faraday constant×powergeneration area (cm²)/8.31×temperature (K)×limiting current (A)  (4)

FIG. 3 is a relationship diagram showing a relationship between thewater absorption rate (%) and the performance decrease rate (%) withregard to the samples #1 to #11. With regard to the samples #3 to #6,and #9 whose water absorption rates were equal to or higher than 11% andequal to or lower than 30%, the performance decrease rates were equal toor lower than 1%. In contrast, with regard to the samples #1, #2, #10and #11 whose water absorption rates were equal to or lower than 8%, andthe samples #7 and #8 whose water absorption rates were equal to orhigher than 40%, the performance decrease rates were equal to or higherthan 4%. Based on the results, it has been found that when the waterabsorption rate of the catalyst electrode layer is equal to or higherthan 11% and equal to or lower than 30% (i.e., the water absorption rateis in a range of 11% to 30%), the durability of the catalyst electrodelayer is increased, as compared to when the water absorption rate isoutside the range.

FIG. 4 is a relational diagram showing a relationship between the waterabsorption rate (%) and the cell resistance (Ω·cm²) with regard to thesamples #1 to #6, and #9 to #11. Based on the results regarding thesamples #1 to #6, and #9 to #11, it has been found that in a range wherethe water absorption rate is equal to or lower than 30%, as the waterabsorption rate increases, the cell resistance decreases. Thus, it hasbeen found that when the water absorption rate of the catalyst electrodelayer is equal to or higher than 11%, the cell resistance is decreased,as compared to when the water absorption rate is lower than 11%.

FIG. 5 is a relationship diagram showing a relationship between thewater absorption rate (%) and the ionomer decomposition rate (%) withregard to the samples #1 to #6, and #9 to #11. Based on the resultsregarding the samples #1 to #6, and #9 to #11, it has been found that inthe range where the water absorption rate is equal to or lower than 30%,as the water absorption rate increases, the ionomer decomposition ratedecreases. This reason is considered to be that as the water absorptionrate of the catalyst electrode layer becomes higher, the decompositionof the ionomer due to hydrogen peroxide generated during the durabilitytest is more suppressed. Thus, it has been found that when the waterabsorption rate of the catalyst electrode layer is equal to or higherthan 11%, the ionomer decomposition rate is decreased, as compared towhen the water absorption rate is lower than 11%.

FIG. 6 is a relationship diagram showing a relationship between thewater absorption rate (%) and the content of Fe (μg/cm²) with regard tothe samples #3 to #6, and #9 to #11. Based on the results regarding thesamples #3, and #9 to #11, it has been found that even when the relativehumidity during the durability test is the same (40% RH), as the contentof Fe in the catalyst electrode layer increases, the water absorptionrate of the catalyst electrode layer decreases. The reason is consideredto be that the decomposition of the ionomer due to hydrogen peroxide ispromoted by Fe contained in the catalyst electrode layer. Thus, it hasbeen found that the content of Fe in the catalyst electrode layer ispreferably equal to or larger than 0 μg/cm² and equal to or smaller than0.14 μg/cm².

FIG. 7 is a relationship diagram showing a relationship between thewater absorption rate (%) and the relative humidity (%) with regard tothe samples #1 to #6, and #9. Based on the results regarding the samples#1 to #6, it has been found that as the relative humidity during thedurability test decreases, the water absorption rate of the catalystelectrode layer decreases. The reason is considered to be that as therelative humidity during the durability test becomes lower, the ionomeris more likely to be decomposed due to hydrogen peroxide during thedurability test. Thus, it has been found that it is preferable to setthe relative humidity to 40% during the durability test, rather thansetting the relative humidity to a value lower than 40%. Further, it ismore preferable to maintain a membrane-electrode assembly for 100 hoursunder the condition that the cell temperature is 60° C., the relativehumidity is 40%, and the generated voltage is 0.5 V during thedurability test.

FIG. 8 is a relationship diagram showing a relationship between thewater absorption rate (%) and the gas diffusion resistance (sec/m) withregard to the samples #3 to #9. Based on the results regarding thesamples #3 to #9 whose water absorption rates were equal to or higherthan 10%, it has been found that as the water absorption rate of thecatalyst electrode layer increases, the gas diffusion resistanceincreases. The reason is considered to be that as the water absorptionrate of the catalyst electrode layer becomes higher, flooding is morelikely to occur due to water retained in the catalyst electrode layer asa result of swelling of the ionomer. Thus, it has been found that whenthe water absorption rate of the catalyst electrode layer is equal to orlower than 30%, the gas diffusion resistance is decreased, as comparedto when the water absorption rate is higher than 30%.

FIG. 9 is a relationship diagram showing a relationship between thewater absorption rate (%) and the performance decrease rate (%) withregard to the samples #12 to #16. With regard to the samples #12 to #14whose water absorption rates were equal to or higher than 11% and equalto or lower than 30%, the performance decrease rates were substantially0%. In contrast, with regard to the sample #15 whose water absorptionrate was 8%, and the sample #16 whose absorption rate was 48%, theperformance decrease rate was 4%. Thus, it has been found that when thewater absorption rate of the catalyst electrode layer is equal to orhigher than 11% and equal to or lower than 30% (i.e., the waterabsorption rate is in the range of 11% to 30%), the durability of thecatalyst electrode layer is increased as compared to when the waterabsorption rate is outside the range. Further, based on the resultsregarding the samples #12 to #16, it has been found that the waterabsorption rate of the catalyst electrode layer is correlated with thedurability of the catalyst electrode layer, regardless of the relativehumidity during the durability test, the materials, and the productionconditions.

Thus, it is considered that high durability (excellent durability) canbe obtained by controlling the water absorption rate of the catalystelectrode layer such that the water absorption rate is in the specifiedrange. More specifically, as evident from the results regarding thesamples #1 to #11 shown in FIG. 3, it is preferable that the waterabsorption rate of the catalyst electrode layer should be controlled tobe equal to or higher than 11% and equal to or lower than 30% (the waterabsorption rate should be controlled to be in the range of 11% to 30%)and the content of Fe in the catalyst electrode layer should becontrolled to be equal to or larger than 0 μg/cm² and equal to orsmaller than 0.14 μg/cm² (the content of Fe should be controlled to bein the range of 0 μg/cm² to 0.14 μg/cm²). When the water absorption rateof the catalyst electrode layer and the content of Fe in the catalystelectrode layer are controlled in the above-mentioned manner, thedurability of the catalyst electrode layer can be improved.

The invention is not limited to the above-mentioned embodiment, and maybe implemented in various modes without departing from the scope of theinvention. For example, the invention may be implemented in modifiedexamples described below.

First Modified Example

In the above-mentioned embodiment, the catalyst electrode layer whosewater absorption rate is 11% to 30%, and whose content of Fe is 0 μg/cm²to 0.14 μg/cm² is used as each of the anode 22 an and the cathode 22 ca.However, only one of the anode 22 an and the cathode 22 ca may beconstituted by the above-mentioned catalyst electrode layer. In thiscase as well, the durability of the catalyst electrode layer can beimproved. It is preferable that both of the anode 22 an and the cathode22 ca should be constituted by the above-mentioned catalyst electrodelayers.

Second Modified Example

The invention may be implemented as a testing method for themembrane-electrode assembly. For example, in the testing method for themembrane-electrode assembly 23 including the catalyst electrode layers22 as shown in FIG. 1, the fuel cell 10 including the membrane-electrodeassembly 23 is prepared, and the durability test is conducted on thefuel cell 10 under the condition that the cell temperature is 60° C. andthe relatively humidity is 40%. By testing the fuel cell 10 in thismanner, it is possible to reduce deterioration of the membrane-electrodeassembly after the test. More specifically, as evident from the resultsshown in FIG. 7, when the relative humidity during the durability testis 40%, the water absorption rate of the catalyst electrode layer is inthe range of 11% to 30%. Thus, it has been found that it is preferableto set the relative humidity during the durability test to 40% such thatthe water absorption rate of the catalyst electrode layer after the testis equal to or higher than 11% and equal to or lower than 30%. Thus, byconducting the durability test during testing, it is possible tosuppress deterioration of the membrane-electrode assembly. The celltemperature during testing is not particularly limited. However, it ispreferable that the cell temperature should be 60° C. The test timeperiod during which the durability test is conducted is not particularlylimited. However, it is preferable that the test time period should be100 hours.

What is claimed is:
 1. A catalyst electrode layer configured to bedisposed in contact with an electrolyte membrane of a fuel cell,wherein: a content of Fe per unit area of the catalyst electrode layeris equal to or larger than 0 μg/cm² and equal to or smaller than 0.14μg/cm², and a water absorption rate of the catalyst electrode layer isequal to or higher than 11% and equal to or lower than 30%; and thewater absorption rate satisfies a relationship of the water absorptionrate=(Q3−Q1)/Q1×100−(Q2−Q1)/Q1×100, where Q1 is a weight of the catalystelectrode layer after the catalyst electrode layer is dried for 1 hourunder an environment in which a temperature is 100° C. and a relativehumidity is 0%, after a fuel cell including the catalyst electrode layeris maintained for 100 hours under a condition that a cell temperature is60° C., the relative humidity is 40%, and a generated voltage is 0.5 V,Q2 is a weight of the catalyst electrode layer after the catalystelectrode layer is further maintained for 1 hour under an environment inwhich the temperature is 70° C. and the relative humidity is 15%, and Q3is a weight of the catalyst electrode layer after the catalyst electrodelayer is further maintained for 1 hour under an environment in which thetemperature is 70° C. and the relative humidity is 90%.
 2. The catalystelectrode layer according to claim 1, comprising catalyst metal; acatalyst support that supports the catalyst metal; and an ionomer,wherein the water absorption rate varies depending on a kind of theionomer, a weight percent of the ionomer in the catalyst electrodelayer, a kind of the catalyst support, a weight percent of the catalystsupport in the catalyst electrode layer, and a structure of the catalystelectrode layer.
 3. A membrane-electrode assembly comprising: anelectrolyte membrane; and the catalyst electrode layer according toclaim 1, the catalyst electrode layer being provided on at least one ofsurfaces of the electrolyte membrane.
 4. A fuel cell comprising themembrane-electrode assembly according to claim 3.